Nonvolatile memory devices are capable of persisting data for extended periods of time without needing to be powered to maintain such data storage. For example, information is written to a nonvolatile flash memory device by changing the electrical characteristics of transistor-based memory cells of the flash memory device to change how such memory cells react to applied voltages. The electrical characteristics of different memory cells in a semiconductor memory device are representative of binary bits that can be read by sensing threshold voltages of the memory cells in response to applied input voltages.
A threshold voltage of a memory cell is the gate-to-source voltage of the memory cell's transistor necessary to allow electrical current to conduct through the source and drain terminals of the transistor. To sense the flow of electrical current and, thus, the threshold voltages of memory cells in a flash memory device, the flash memory device is provided with sense amplifiers in circuit with columns of the memory cells. In this manner, the sense amplifiers can be used to latch logical values corresponding to the electrical current flowing through the memory cells. The latched logical values can then be processed to determine threshold voltages of memory cells. In turn, the threshold voltages are indicative of different binary bit values stored in the memory cells. Such sensing operations can be used to read programmed states of the memory cells during program-verify operations used to confirm whether intended information (e.g., data) is fully written to memory cells during write operations. Sensing operations are also used during normal read operations of the memory cells that can be performed one or more times after information is successfully written to the memory cells.